As a result of the relatively high cost of oak barrels, winemakers have been searching for a comparable alternative at a fraction of the cost. Oak chips, while somewhat taboo in their earlier days, are gaining favor among winemakers as being a good alternative to the traditional oak barrel. In blind taste tests, consumers showed no preference for wine aged in oak barrels versus wine aged with oak chips. For the details of that particular study, click here to read a past review by The Academic Wino.
Producing staves for barrel or oak chips is not a fast process. One of the lengthiest steps is the outdoor seasoning process, which takes anywhere from 24 to 36 months. It is during this time that the wood undergoes many biochemical transformations of biopolymers and other compounds by fungi and bacteria. Studies have found the fungi population present during this seasoning process consists of Aureobasidium pullulans (83%), Trichoderma harzianum, and Trichoderma konigii (the latter two making up 15% of the population). These fungi function to hydrolyze wood heterosides (including ellagitannins, coumarins, and polysaccharides) which result in a decrease in bitterness and astringency.
Over the 24-36 month seasoning time, the fungal community changes. Over this time, fungi belonging to the genera Penicillum, Geomyces, and Geotrichum, with the species Penicillum purpurogenum the most represented. In the internal layers of the staves, studies have found that the species Candida sp., Paecilomyces variotii, and Phialemonium sp. were the most represented molds.
Some scientists have hypothesized that by inoculating oak staves with certain fungi, they may be able to better control the metabolic reactions and therefore which wood compounds are hydrolyzed. This could potentially lead to new seasoning places and greater customization of the desired flavors in the wine that is aged in a particular barrel or with particular oak chips. Some studies have reported that by inoculating oak staves with fungi have increased the seasoning rate, thereby dropping the wait time for a finished staves or oak chips from 12-36 months to just one month.
The goal of the paper reviewed today, which was published last year, was to treat oak chips with certain combinations of fungi in order to potentially improve the impact of oak chips in red wine maceration, and to obtain effects comparable to wine aged in oak barrels.
The following fungi were used in this experiment: Ph. chrysosporium Burds. (MUT 2660), P. purpurogenum Stoll (MUT 3316), A. pullulans (de Bary) G. Arnaud (MUT 3237), and Phi. obovatum W. Gams & McGinnis (MUT 2702). The fungi used were all non-mycotoxinogenic. After an incubation period, an agar plug (8mm in diameter from along the edge of an actively growing colony) of each fungus was used to prepare and inoculate the oak chips. Each preparation used a different combination of fungi and growth medium.
Oak chips were toasted at a low degree and were of medium size. A 3mL aliquot of fungi and growth medium preparations was added to a flask containing 4g of oak chips and either 12mL of laboratory medium or 12mL of saline solution. Oak chips were sterilized either with the laboratory medium or the saline solution.
These cultures were incubated for 12 weeks in the dark under static conditions at temperatures optimized for each type of fungus. In addition to each fungus by itself, a combination of A. pullulans and Ph. chrysosporium was studied. For this combination, oak chips were first inoculated with A. pullulans for 6 weeks, then sterilized, then inoculated with Ph. chrysosporium for another 6 weeks.
Following the incubation period, oak chips were removed and brushed off to remove visible pieces of debris. The chips were then used for aging of red wine using traditional winemaking processes. The two grape varieties used were Montepulciano d’Abruzzo (70%) and Merlot (30%) from the 2006 vintage and originating from vineyards in San Severo, Apuila (southern Italy). Artificial aging was done by placing 1g of oak chips in 500mL bottles containing the wine and storing them in a 20oC room (82% relatively humidity) for 17 days.
The following volatiles were measured and analyzed: furfural, furfuryl alcohol, guaiacol, syringol, cis-β-methyl-γ-octalactone, 2-phenylethanol, 4-vinylguaiacol, benzyl alcohol, 2,3-butanediol, γ-butyrolactone, benzylaldehyde, and 4-ethylguaiacol. Gallic acid and ellagic acid were also analyzed.
- Furfural, furfuryl alcohol, guaiacol, syringol, cis-β-methyl-γ-octalactone are frequently present in wine after oak aging.
- 2-phenylethanol and 4-vinylguaiacol are known fermentation products, and have been shown to increase after oak aging.
- 4-ethylguaiacol is associated with Brettanomyces or Dekkera infections, and is associated with characteristic flavors such as “bacon” or “smoked”.
- Cis-β-methyl-γ-octalactone is associated with oaky characteristics such as coconut and vanilla.
- Furfural contributes to characters such as “dried fruits” and “burned almonds”. Studies have shown it does not play an important role in the aroma of wine, though it may strengthen the aroma of lactones.
- Guaiacol contributes to “burnt” overtones in wine aroma.
- Syringol is an indicator of the relative toast of the oak wood. Compared to guaiacol, it has a weak odor and relatively little impact on the flavor of the wine.
- Benyzlaldehyde is associated with a “bitter almond” aroma.
- 2,3-butanediol is odorless, though still contributes to the sweet taste of a wine.
- Cis-β-methyl-γ-octalactone was present in all wine samples.
- The presence and concentrations of specific volatile compounds and phenols were influenced by the type of fungus and medium used for the particular oak chip treatment.
- Concentrations of furfural were significantly affected by Ph. chrysosporium and P. purpurogenum by increasing in both laboratory medium and saline solution treatment. Perception threshold was not reached.
- There were no significant changes with any other treatment.
- Furfural appears to be the most susceptible oak wood volatile compound to microbial transformations.
- Fungal treatment of the oak chips resulted in a significant increase in the concentration of guaiacol.
o For the saline solution, this increase was found in the treatments inoculated with P. purpurogenum, A. pullulans, and Phi. obovatum.
o For the laboratory medium, this increase was found in the treatments inoculated with P. purpurogenum and A. pullulans.
o For treatments with A. pullulans, guaiacol was above the perception threshold.
- For the saline solution, there was an increase of syringol concentrations with the treatments inoculated with P. purpurogenum.
- For the laboratory medium, higher levels (above perception threshold) of syringol were found in the treatments inoculated with P. purpurogenum and A. pullulans.
- For the saline solution, fungal treatment decreased the concentrations of benzylaldehyde (except the treatment with Ph. chrysosporium).
- For the laboratory medium, there was a significant increase in benzylaldehyde concentrations.
- For both the saline solution and laboratory medium, there was a decrease in 2,3-butanediol levels in all fungal treatments except samples treated with A. pullulans.
Ellagic and Gallic Acids
- Ellagic acid and gallic acid were metabolized by the fungi, with the exception of wines treated with P. purpurogenum in the laboratory medium.
What do these results means?
The results of this study show that the fungal treatment of the oak chips significantly affected the chemical profile of the wine. Based on principle components statistical analysis, the results showed two distinct groups that affected wines in specific ways. Oak chips treated with Phi. obovatum, A. pullulans, and the combination of A. pullulans and Ph. chrysosporium (“Group A”) affected the chemical profile of wines in one particular way, and oak chips treated with P. purpurogenum and Ph. chrysosporium affected the chemical profile of wines in another way (“Group B”).
Group A treated wines showed increases in guaiacol and syringol concentrations, whereas Group B treated wines showed increases in furfural and benzylaldehyde concentrations.
Even though the effect of the fungi were variable depending upon what kind of medium was used for the chips (laboratory medium versus saline solution), Group B treated wines showed increases in furfural and benzylaldehyde regardless of the medium the chips were treated with.
Based on these results, the authors claim that the microfungal treatment of oak chips increases the concentrations of some volatile components in red wine during aging. It may be possible, that with fungal treatment of oak chips for the aging of red wines, to tailor the flavors and aromas to those desired for a particular style of finished wine. If the goal is to have a wine with greater “toasty” character, the use of fungi from Group A may be useful. If the goal is to have a wine with more “dried fruit” or “almond” character, then a fungi of Group B may be better.
Of course, this research is in its infancy, and more work would need to be done, particularly in regard to examining the biology and enzymatic profile of the fungi, and any potential positive or negative health consequences of using it in the aging of wines.
I’d love to hear what you all think of the use of fungi in extracting more oak character from oak chips in the aging of wine. Please feel free to leave your comments below!
Source: Petruzzi, L., Bevilacqua, A., Ciccarone, C., Gambacorta, G., Irlante, G., Pati, S., and Sinigaglia, M. 2010. Use of microfungi in the treatment of oak chips: possible effects on wine. Journal of the Science of Food and Agriculture 90: 2617-2626.
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!